JP4007241B2 - Austenitic stainless steel excellent in high-temperature strength and corrosion resistance, heat-resistant pressure-resistant member made of this steel, and manufacturing method thereof - Google Patents

Description

【００７０】
前記の工程Ａ、ＢまたはＣによって得られた熱間加工鋼板、冷間圧延鋼板および冷間加工鋼管について、オーステナイト結晶粒度番号と混粒率を調べた。オーステナイト結晶粒度番号は、ASTMに規定される方法に従って測定し、混粒率は前述した方法により求めた。その際、いずれの場合も20視野を観察した。
【００７１】
同じく、工程Ａ、ＢまたはＣによって得られた熱間加工鋼板、冷間圧延鋼板および冷間加工鋼管から、外径6mm、標点距離30mmのクリープ試験片を採取してクリープ試験に供し、750℃、10,000時間のクリープ破断強度（MPa）と絞り率（内挿値：％）を調べた。以上の結果を、表２にまとめて示す。
【００７２】
【表２】

【００７３】
表２からわかるように、化学組成が本発明で規定する範囲内の鋼（Ｎｏ．１、２、４、６、７、９、１１〜１３、１６および１８〜２０）では、Ａ、Ｂ、Ｃのどの方法で加工しても、オーステナイト結晶粒度番号と混粒率が本発明で規定する範囲になっている。その結果、７５０℃、１０，０００時間のクリープ破断強度が８７ＭＰａ以上、絞り率が６０％以上と高く、耐熱疲労特性と組織安定性に優れた耐熱耐圧部材が得られることが明らかである。
【００７４】
No.21（SUS310）およびNo.22（SUS316）は、組織は本発明で規定する条件を満たす粗粒組織になっているが、化学組成が本発明で規定する範囲外であるため、クリープ破断強度がそれぞれ41MPaおよび55MPaと著しく低い。
【００７５】
化学組成が本発明で規定する範囲外の鋼（No.23〜29）では、本発明の製造方法により加工熱処理しても、オーステナイト結晶粒度番号と混粒率の両方が本発明で規定する範囲内にある粗粒組織は得られていない。その結果、クリープ破断強度が68〜78MPa、絞り率が4〜23％と低い。No.25はＯ（酸素）含有量が高すぎ、また、No.26はＮの含有量が高すぎるものである。No.29は、Ｏ含有量およびＮ含有量が両方とも高すぎる。これらのクリープ破断強度および絞り率が目標値をはるかに下回ることから、ＯとＮの含有量を低く抑えることの重要性がわかる。即ち、これらの比較鋼では、700℃以上の高温域において優れた耐熱疲労特性と組織安定性を発揮する耐熱耐圧部材は得られない。
【００７６】
【発明の効果】
本発明のオーステナイト系ステンレス鋼は、高温強度と高温耐食性が良好なだけでなく、オーステナイト結晶粒度番号と混粒率がそれぞれ６以下、10％以下の粗粒組織で、700℃以上の高温域での耐熱疲労特性と組織安定性に優れた耐熱耐圧部材の素材として好適である。また、本発明の耐熱耐圧部材は、750℃、10,000時間のクリープ破断強度が87MPa以上、絞り率が57％以上と高いので、蒸気温度が700℃以上というような超々臨界圧ボイラ等の構成部材として使用可能である。さらに、本発明の方法によれば、本発明の耐熱耐圧部材を低コストで製造することが可能である。[0001]
BACKGROUND OF THE INVENTION
The present invention is an austenitic stainless steel suitable as a material for steel pipes, steel plates, bar steels, forged steel products, etc. (hereinafter collectively referred to as “heat-resistant pressure-resistant members”) constituting power generation boilers, heating furnaces for chemical industries, and the like. The present invention relates to a heat and pressure resistant member made of the steel and excellent in high temperature strength and high temperature corrosion resistance, and a method for producing the same. In addition to having high high-temperature strength and excellent high-temperature corrosion resistance, this heat-resistant pressure-resistant member is also excellent in heat fatigue resistance and metal structure stability (hereinafter simply referred to as structure stability).
[0002]
[Prior art]
In recent years, new super-critical pressure boilers with higher steam temperature and pressure have been developed all over the world for higher efficiency. The steam temperature is planned to increase from around 600 ° C to 650 ° C or higher, and to 700 ° C or higher in the future. This is the CO for energy conservation, effective use of resources, and environmental conservation.₂Reducing gas emissions has become a major issue, and a high-efficiency ultra-supercritical boiler that burns fossil fuel is advantageous in solving this problem.
[0003]
The high temperature and high pressure of the steam, especially the high temperature, raises the temperature of the heat-resistant pressure-resistant member constituting the heating furnace for the boiler and chemical industry, and the temperature reaches 650 ° C. or more. For this reason, in addition to high temperature strength and high temperature corrosion resistance, these heat and pressure resistant members are required to have heat fatigue characteristics and long-term structural stability.
[0004]
Austenitic stainless steel is superior in high-temperature strength and high-temperature corrosion resistance compared to ferritic steel. For this reason, austenitic stainless steel is used in a high temperature range of 650 ° C. or higher where ferritic steel cannot be used from the viewpoint of strength and corrosion resistance.
[0005]
As austenitic stainless steels for high temperature and high pressure, 18-8 austenitic stainless steels such as SUS347H and SUS316 are widely used, but there are limits in high temperature strength and corrosion resistance. There is also 25Cr SUS310 with improved corrosion resistance, but its high-temperature strength at 600 ° C or higher is lower than SUS316.
[0006]
For this reason, many steels having high temperature strength and high temperature corrosion resistance based on 20Cr or higher austenitic stainless steel having corrosion resistance higher than 18-8 series steel have been proposed. These steels can be divided into the following three categories.
[0007]
(1) Steel in which the Cr content is increased to 20% or more and the solid solution strengthening elements W, Mo, etc. are added in combination to enhance the intragranular strengthening (for example, Japanese Patent Laid-Open Nos. 61-179833 and 61) -179835).
[0008]
(2) Steel in which N is positively added in addition to W and Mo to enhance precipitation strengthening with nitrides (for example, JP-A-63-183155).
[0009]
(3) Steel that is precipitation strengthened by an intermetallic compound of Ti or Al (for example, JP-A-7-216511).
[0010]
However, the steel of the above (1) has a low high-temperature creep strength at 700 ° C. or higher because the main component of creep in the high-temperature region is from intergranular dislocation creep to intergranular slip creep. Although the steels (2) and (3) have sufficient strength, the ductility is remarkably low, the heat fatigue characteristics and the structural stability at high temperatures are inferior, and the creep strength and creep ductility at 700 ° C. or higher are low.
[0011]
The steel of (3) has high strength and toughness due to intergranular sliding creep and non-uniform creep deformation due to the mixed grain structure because the intermetallic compound of Ti and Al suppresses the growth of crystal grains. Damaged. Therefore, these conventional steels cannot be used as a heat and pressure resistant member used at a high temperature of 700 ° C. or higher, particularly as a heat resistant and pressure resistant member having a thickness of 20 mm or more, which tends to be a mixture with significant structure.
[0012]
[Problems to be solved by the invention]
A first object of the present invention is to provide an austenitic stainless steel suitable for a material of a heat and pressure resistant member exhibiting excellent heat fatigue characteristics and structural stability in a high temperature range of 700 ° C. or higher.
[0013]
The second object of the present invention is to provide a heat and pressure resistant member excellent in high temperature strength and heat fatigue resistance. In particular, the object is to provide a heat and pressure resistant member having the characteristics that the creep rupture strength and drawing rate at 750 ° C. for 10,000 hours are 80 MPa or more and 55% or more, respectively.
[0014]
A third object of the present invention is to provide a method for producing a heat and pressure resistant member having the above-mentioned characteristics.
[0015]
[Means for Solving the Problems]
The austenitic stainless steels of the present invention are the following steels (1) and (2). The heat-resistant pressure-resistant member of the present invention is the following member (3). Furthermore, the production method suitable for obtaining the heat and pressure resistant member of the present invention is the method (4) below.
[0016]
  (1) By mass%, C: 0.03-0.12%, Si: 0.1-1%, Mn: 0.1-2%, Cr: 20% or more and less than 28%, Ni: 35% Exceeding 50% or less, W: 4 to 10%, Ti: 0.01 to 0.3%, Nb: 0.01 to 1%, sol. Al: 0.0005-0.04%, B: 0.0005-0.01% included, the balance is made of Fe and impurities, P as impurities is 0.04% or less, S is 0.010% or less , Mo is less than 0.5%, N is less than 0.02%, O (oxygen) is 0.005% or lessA coarse grain structure having an austenite grain size number of 6 or less and a mixed grain ratio of 10% or less.An austenitic stainless steel characterized by
[0017]
  (2) In addition to the component described in (1) above, it further includes at least one component selected from at least one of the following first group to third group, the balance being Fe and impurities, P 0.04% or less as impurities, S 0.010% or less, Mo less than 0.5%, N less than 0.02%, O (oxygen) 0.005% or lessA coarse grain structure having an austenite grain size number of 6 or less and a mixed grain ratio of 10% or less.An austenitic stainless steel characterized by
First group: Zr of 0.0005 to 0.1% by mass%.
Second group: 0.0005 to 0.05% Ca by mass%.
Third group: 0.0005 to 0.2% of rare earth elements, Hf and Pd, respectively, by mass%.
[0018]
Here, the rare earth elements are 15 elements from La of the original number 57 to Lu of the same 71, and 17 elements including Y and Sc.
[0019]
  (3) From the austenitic stainless steel described in (1) or (2) aboveBecomeA heat and pressure resistant member excellent in heat fatigue characteristics and structural stability at high temperatures. In particular, a heat and pressure resistant member having a creep rupture strength and a drawing ratio of 750 ° C. for 10,000 hours and 80 MPa or more and 55% or more, respectively.
[0020]
Here, the above-mentioned austenite crystal grain size number means a grain size number defined by ASTM (American Society for Testing and Material).
[0021]
Next, a method for calculating the mixing rate (%) will be described. Assuming that the number of fields observed when determining the austenite grain size number with an optical microscope is N, by counting the number of crystal grains present in each field of view, the austenite grain size number is -3 (coarse) To +10 (fine grain), it is determined that the particle number is N, N determination results are obtained, and the frequency is calculated for each particle number. Then, the particle size number G having the maximum frequency is specified, the number of visual fields n1 having a particle size number 3 or less than the specified particle size number G, and the number of visual fields n2 having a particle size number 3 or more larger than the specified particle size number G. And ask. The percentage obtained by dividing the total number of the visual field numbers n1 and n2 by the total visual field number N, that is, 100 × (n1 + n2) / N is the mixed particle ratio.
[0022]
  (4) The steel having the chemical composition as described in (1) or (2) above is subjected to the following steps(i),(ii)and(iii)The method for producing a heat and pressure resistant member excellent in heat fatigue resistance and structure stability in a high temperature range as described in the above (3), wherein the heat treatment and pressure resistance member are excellently processed.
Process(i): Heat to 1100 ° C. or more at least once before final processing by hot or cold.
Process(ii): Plastic working with a cross-section reduction rate of 10% or more.
Process(iii): Final heat treatment is performed at 1050 ° C. or higher.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have examined in detail the influence of alloying elements on the creep and metal structure of austenitic stainless steel with a Cr content increased to 20% or more to ensure corrosion resistance in a high temperature range at 700 ° C. or higher. As a result, the following new findings were obtained.
[0024]
(A) Mo not only has little effect on increasing the strength at high temperatures above 700 ° C, but also reduces high-temperature corrosion resistance, so even if it is included as an impurity, its content is limited to less than 0.5%. There is a need.
[0025]
(B) Unlike Mo, W improves strength in the high temperature range of 700 ° C and higher, and does not reduce high temperature corrosion resistance. Insufficient addition of Mo results in insufficient addition of W. Can compensate.
[0026]
(C) As described above, carbonitrides and intermetallic compounds containing a large amount of Ti, which are used to increase the strength in the prior art, promote intergranular sliding creep and non-uniform creep deformation, resulting in high temperatures. It is better not to use as much as possible because it will significantly reduce the strength and ductility in the region.
[0027]
(D) Grain boundary sliding creep and non-uniform creep deformation are less likely to occur in the coarse-grained structure than in the fine-grained structure. It is less likely to occur in the case of a structure, preferably a coarse structure with a mixed particle ratio of 0 (zero).
[0028]
  (E) A coarse grain structure having an austenite grain size number of 6 or less and a mixed grain ratio of 10% or less restricts the Ti content in steel to 0.01 to 0.3%, and N and O (oxygen) Of the chemical composition according to (1) or (2), wherein the content of each of the above is limited to less than 0.02% and 0.005% or less, and an appropriate amount (0.0005 to 0.01%) of B is contained. Steel is used as a raw material, and this steel is used in the above process(i)From(iii)Can be obtained through the process.
[0029]
  That is, when the content of Ti, N, O and B is limited to the above range,(i)There is no undissolved carbonitride or oxide containing stable Ti or B in the steel after the process(ii)Uniform strain is accumulated in the process(iii)In this case, recrystallization proceeds uniformly to obtain a heat and pressure resistant member having a coarse grain structure with an austenite grain size number of 6 or less and a mixed grain ratio of 10% or less.
[0030]
(F) The above amounts of Ti and Nb are grains as fine carbides during creep in the case of actually using a heat and pressure resistant member whose structure is an austenite grain size number 6 or less and a mixed grain ratio 10% or less. Precipitates uniformly inside and at the grain boundaries, improving high-temperature creep strength. As a result, the creep rupture strength at 750 ° C. and 10,000 hours of this member is 80 MPa or more, and the drawing ratio is 55% or more. A member having such characteristics is also excellent in heat fatigue resistance.
[0031]
  Hereinafter, the chemical composition of the austenitic stainless steel of the present invention,Grain size and blending ratio,Heat resistant and pressure resistant member made of this steelThatThe reason why various conditions of the preferred manufacturing method are determined as described above will be described in detail. In the following, “%” means “mass%” unless otherwise specified.
[0032]
1. Chemical composition of austenitic stainless steel
C: 0.03-0.12%
C is a component necessary for forming high-temperature tensile strength and high-temperature creep strength necessary for high-temperature austenitic stainless steel by forming carbides, and a content of 0.03% or more is necessary. However, if the content exceeds 0.12%, undissolved carbides are produced, or Cr carbides increase and weldability decreases, so the upper limit was made 0.12%. A desirable C content is 0.05 to 0.10%.
[0033]
Si: 0.1-1%
Si is added as a deoxidizer during steelmaking, but it is also an element necessary for improving the steam oxidation resistance of steel, and a content of at least 0.1% is necessary. However, if the content is excessive, the workability of steel deteriorates, so the upper limit was made 1%. A preferred range is 0.1-0.5%.
[0034]
Mn: 0.1-2%
Mn combines with impurity S contained in the steel to form MnS and improves hot workability, but if its content is less than 0.1%, this effect cannot be obtained. On the other hand, if the content is excessive, the steel becomes hard and brittle, and on the contrary, the workability and weldability are impaired, so the upper limit was made 2%. A desirable Mn content is 0.5 to 1.2%.
[0035]
P: 0.04% or less
P is inevitably mixed as an impurity, but excess P impairs weldability and workability, so the upper limit is made 0.04%. A preferred upper limit is 0.03%. The smaller the P content, the better.
[0036]
S: 0.010% or less
S is inevitably mixed as an impurity as in the case of P described above, but excessive S impairs weldability and workability, so the upper limit is made 0.010%. A preferred upper limit is 0.008%. The S content is preferably as low as possible in order to improve the workability, but it is preferable to contain about 0.004 to 0.008% in order to ensure the hot water flow during welding.
[0037]
Cr: 20% or more, less than 28%
Cr is an important element for ensuring oxidation resistance, steam oxidation resistance and corrosion resistance. A minimum content of 20% is required to make the corrosion resistance at high temperatures of 700 ° C or higher 18 or higher. The corrosion resistance is improved as the Cr content is increased. However, when the Cr content is 28% or more, the structural stability is lowered and the creep strength is deteriorated. Moreover, not only is it necessary to increase the expensive Ni content in order to stabilize the austenite structure, but also the weldability is reduced. Therefore, the Cr content is 20% or more and less than 28%. A preferred range is 22-26%.
[0038]
Ni: Over 35% and 50% or less
Ni is an element that stabilizes the austenite structure, and is an alloy element that is also important for ensuring corrosion resistance. In view of the balance with the above Cr amount, Ni needs to exceed 35%. On the other hand, excessive Ni causes not only an increase in cost but also a decrease in creep strength, so the upper limit is made 50%. Desirable is 40-48%.
[0039]
Mo: Less than 0.5%
As described above, Mo not only causes an embrittlement phase and deteriorates corrosion resistance in an environment of use at 700 ° C. or higher, but combined addition with W, which will be described later, has an effect of improving strength compared to the addition of W alone. rare. For this reason, Mo is not actively added in the present invention. However, even if the amount of impurities is 0.5% or more, when used in a high temperature range of 700 ° C. or higher, the formation of an embrittled phase and a decrease in corrosion resistance become significant. Therefore, the Mo content as an impurity is set to less than 0.5%. It is preferably 0.3% or less, more preferably less than the detection limit value of the analysis. The detection limit value of Mo is usually 0.01%.
[0040]
W: 4-10%
W is one of the important elements and suppresses grain boundary sliding creep which is preferential in a high temperature range of 700 ° C. or higher by the solid solution strengthening action. For this purpose, a content of at least 4% is required. On the other hand, although excessive W does not generate an embrittlement phase like Mo, the steel is markedly hardened and deteriorates workability and weldability, so the upper limit is made 10%. 6-8% is desirable.
[0041]
Ti: 0.01-0.3%
Ti forms undissolved carbonitrides and oxides and promotes the mixing of austenite grains, and causes uneven creep deformation and reduced ductility. Therefore, its content is 0.3% or less. did. On the other hand, if the content is less than 0.01%, improvement in high temperature strength due to precipitation of carbide during use in a high temperature region cannot be expected. For this reason, Ti content was made into 0.01 to 0.3%. Preferred is 0.03 to 0.2%.
[0042]
Nb: 0.01-1%
Nb does not become a harmful oxide like Ti, but a minimum content of 0.01% is necessary to improve the creep strength due to carbide. On the other hand, excessive Nb impairs weldability, so the upper limit is made 1%. Preferred is 0.1 to 0.5%.
[0043]
sol.Al: 0.0005-0.04%
Al is added as a deoxidizer, but if added in a large amount, the stability of the structure deteriorates, so the content thereof is 0.04% or less in terms of sol.Al content. On the other hand, in order to obtain a sufficient deoxidation effect, a sol.Al content of 0.0005% or more is necessary. The preferred range is 0.005 to 0.02%.
[0044]
B: 0.0005-0.01%
B is an element having an extremely effective grain boundary slip creep inhibiting effect in the steel of the present invention in which the content of N and O (oxygen) described later is reduced to eliminate oxides and nitrides as much as possible. However, if the content is less than 0.0005%, this effect cannot be obtained. On the other hand, if it exceeds 0.01%, weldability is impaired. Therefore, the B content is set to 0.0005 to 0.01%. The preferred range is 0.001 to 0.005%.
[0045]
N: Less than 0.02%
One of the important requirements of the present invention is to reduce the content of N and O described below. Conventionally, N is positively added as an element replacing precipitation strengthening by carbonitride and a part of expensive Ni. However, a large amount of N produces undissolved carbonitrides of Ti and B, which mixes the structure and promotes intergranular slip creep and non-uniform creep deformation at high temperatures of 700 ° C and higher. Damage. Therefore, it is necessary to reduce the N content as much as possible. N has a strong affinity for Cr, and it is inevitable that it is mixed as an impurity. However, if the content is less than 0.02%, the undissolved carbonitride is not generated, so the N content is less than 0.02%. It is preferably 0.016% or less, more preferably 0.01% or less. The lower the N content, the better.
[0046]
O (oxygen): 0.005% or less
O, like N above, produces insoluble oxides of Ti and Al, which mixes the structure and promotes intergranular slip creep and non-uniform creep deformation at high temperatures above 700 ° C. And lose strength. Therefore, it is necessary to reduce the O content as much as possible. Although it is inevitable that O is also mixed as an impurity, if the content is 0.005% or less, the undissolved oxide is not generated, so the O content is set to 0.005% or less. It is preferably 0.003% or less. The lower the O content, the better.
[0047]
The balance of the austenitic stainless steel of the present invention is substantially Fe, in other words, Fe and impurities other than those described above.
[0048]
Another of the austenitic stainless steels of the present invention is a steel containing at least one component selected from at least one group from the first group to the third group. Hereinafter, these components will be described.
[0049]
First group (Zr)
Zr has the effect of strengthening grain boundaries and improving high temperature strength. Therefore, when it is desired to obtain the effect, it may be actively added and contained. The effect becomes remarkable when the content is 0.0005% or more. However, if its content exceeds 0.1%, it forms undissolved oxides and nitrides similar to Ti, and not only promotes intergranular sliding creep and non-uniform creep deformation, but also improves the steel quality. Deteriorate the creep strength and ductility at high temperatures. For this reason, the Zr content when added is preferably 0.0005 to 0.1%. More preferred is 0.001 to 0.06%.
[0050]
  Second group (Ca)
  Ca is,Combined with S, it stabilizes S and has an effect of improving workability. Therefore, if you want to get the effectCaMay be added actively in that case.,The above effect becomes remarkable at a content of 0.0005% or more. However,If it exceeds 0.05%, toughness, ductility and steel quality are impaired. Therefore, the Ca content when added is 0.0005 to 0.05%.WhenIt is good to do. More preferable Ca content is 0.0005 to 0.01%.sois there.
[0051]
Group 3 (rare earth elements, Hf and Pd)
All of these elements have the effect of forming harmless and stable oxides and sulfides, reducing the undesirable effects of O and S, and improving the corrosion resistance, workability, creep strength and creep ductility. Therefore, when it is desired to obtain the effect, one or more kinds may be positively added and contained, and in this case, the above effect becomes remarkable at a content of 0.0005% or more. However, when the content exceeds 0.2%, inclusions such as oxides increase, which not only impairs workability and weldability, but also increases costs. Therefore, the content of these elements when added is preferably 0.0005 to 0.2%. More preferable ranges are 0.001 to 0.1%, respectively.
[0052]
In addition, as impurities other than P, S, Mo, N, and O, Co and Cu which may be mixed from scrap etc. are mentioned. However, Co does not have a special adverse effect on the properties of the steel and the heat and pressure resistant member of the present invention. Therefore, the Co content when mixed as an impurity is not particularly limited. However, since Co is also an activation element, the Co content when mixed is 0.8% or less, preferably 0.5% or less.
[0053]
Although Cu improves strength, it significantly promotes grain boundary sliding creep at high temperatures of 700 ° C and higher. Therefore, the Cu content when mixed as an impurity is 0.5% or less, preferably 0.2% or less.
[0054]
  2.Austenitic stainless steel andHeat and pressure resistant materialMetallographic structure
  Of the present inventionAustenitic stainless steel andHeat and pressure resistant materialofThe metal structure should be a coarse grain structure with an austenite grain size number of 6 or less and a mixed grain ratio of 10% or less. The reason is as follows.
[0055]
As described above, the creep strength at a high temperature range of 700 ° C. or more greatly depends on the size of the austenite crystal grains and the degree of sizing. In the case of a fine grain structure having a grain size number exceeding 6, the grain boundary sliding creep is Arise. Further, even if the grain size number is 6 or less, if the mixing rate exceeds 10%, non-uniform creep deformation occurs. As a result, heat fatigue characteristics and structural stability are poor, and it is impossible to secure 80 MPa or more at a creep rupture time of 10,000 hours at 750 ° C. and 55% or more in a drawing ratio.
[0056]
Therefore, in the present invention, the austenite grain size number is set to 6 or less, and the mixed grain ratio is set to 10% or less. A preferred austenite grain size number is 5.5-3. Further, the preferable mixing ratio is 0 (zero)%, that is, a coarse and sized structure having a particle size number of 6 or less. The lower limit of the austenite grain size number is not particularly limited. However, the coarse grain structure having a grain size number of less than 0 cannot be inspected for internal defects and surface defects by the ultrasonic flaw detection method. Is good.
[0057]
  3. Manufacturing method for heat and pressure resistant members
  Next, a preferable manufacturing method for obtaining the heat and pressure resistant member of the present invention having a coarse grain structure with the austenite grain size number of 6 or less and a mixed grain ratio of 10% or less will be described. This manufacturing method is described above.(i)From(iii)It is characterized by sequentially going through the steps up to.
[0058]
  Process(i):
  In the method of the present invention, it is necessary to perform at least one heating before the final processing by hot or cold so that precipitates in the steel precipitated during the processing are sufficiently dissolved. However, when the heating temperature is less than 1100 ° C., undissolved carbonitrides and oxides containing stable Ti and B are present in the heated steel. As a result, this is the next step(ii)Causes non-uniform distortion to accumulate in the process(iii)In the final heat treatment, recrystallization is made non-uniform. In addition, the undissolved carbonitride or oxide itself inhibits uniform recrystallization, and the predetermined coarse grain structure cannot be secured. For this reason, in the preferred method of the present invention, heating is performed at 1100 ° C. or more at least once before the final processing by hot or cold. The upper limit of the heating temperature is not particularly limited, but heating to a temperature exceeding 1350 ° C. may cause high-temperature intergranular cracking and a decrease in ductility, so the upper limit of the heating temperature is preferably 1350 ° C.
[0059]
You may perform the final process by hot or cold immediately after a heating. There are no special restrictions on the cooling conditions after the heating and when the final processing is hot working. However, it is desirable to cool between 800 ° C. and 500 ° C. at a cooling rate of 0.25 ° C./second or more. This is because coarse precipitates are not formed during cooling.
[0060]
  Process(ii):
  Process(ii)The plastic working process(i)When the final processing in is hot processing, it may be either hot processing or cold processing including warm processing at a temperature of 500 ° C. or less. Also, process(i)In the case of cold working including warm working at a temperature of 500 ° C. or less, the cold working under the same conditions as the final working is used.
[0061]
The plastic working in this step is performed for the purpose of imparting strain in order to promote recrystallization in the next final heat treatment. When the cross-section reduction rate of this processing is less than 10%, the strain necessary for recrystallization cannot be imparted, and the desired coarse grain structure cannot be obtained even if the next final heat treatment is performed. For this reason, the plastic working is performed at a cross-section reduction rate of 10% or more. A desirable lower limit of the cross-section reduction rate is 20%. Note that the larger the cross-section reduction rate, the better, so the upper limit is not specified, but the maximum value in normal processing is about 90%. This processing step is also a step of determining the dimensions of the product.
[0062]
  Process(iii):
  Heat treatment for obtaining a desired coarse grain structure. When the heating temperature of this heat treatment is lower than 1050 ° C., sufficient recrystallization does not occur and a desired coarse grain structure cannot be obtained. Further, the crystal grains become a flat processed structure, and the creep strength is lowered. For this reason, the final heat treatment is performed at 1050 ° C. or higher. The preferred heat treatment temperature is the process(i)The temperature is 10 ° C. or more lower than the heating temperature at. The upper limit of the final heat treatment temperature is not particularly limited.(i)For the same reason as above, the upper limit is preferably 1350 ° C. In addition, after the final heat treatment,(i)For the same reason as above, it is desirable to cool between 800 ° C. and 500 ° C. at a cooling rate of 0.25 ° C./second or more.
[0063]
【Example】
  It has the chemical composition shown in Table 1.22Various types of steel were melted. In addition, No. in the comparative example. 21 is a steel equivalent to SUS310, No. 21. 22 is a steel equivalent to SUS316.
[0064]
  No. 12, 4, 6, 7, 9, 11-13, 16 and 18The steel of ˜20 was melted into a steel ingot using a vacuum melting furnace with a capacity of 50 kg. And No. 12, 4, and 11-13The steel ingot of No. 1 is finished into a plate by the following production method A.6-7 and No.16The steel ingot was finished into a cold-rolled sheet by the following production method B. No.9And No. Steel ingots 18 to 20 were finished into steel pipes by the following production method C.
[0065]
Steel Nos. 21 to 29 were melted using a vacuum melting furnace having a capacity of 150 kg, and each steel ingot obtained was treated by the following production methods A, B and C as shown in Table 2. These production methods all belong to the production method of the present invention.
[0066]
  (1) Manufacturing method A
  Process 1 (Process(i)Equivalent): heated to 1220 ° C,
Process 2 (Process(ii)Equivalent to :) 25 mm thick plate by hot forging with a cross-section reduction rate of 67%
Molded into,
Step 3: Cool from 800 ° C. to 500 ° C. or less at 0.55 ° C./second,
Process 4 (Process(iii)): Held at 1210 ° C. for 15 minutes and then water-cooled.
[0067]
  (2) Production method B
  Process 1 (Process(i)Equivalent): heated to 1220 ° C,
Process 2: Formed into a plate with a thickness of 25 mm by hot forging with a cross-section reduction rate of 67%.
Step 3: Cool from 800 ° C to 500 ° C or less at 0.55 ° C / second
Process 4: Formed into a 20 mm thick plate by external cutting,
Process 5 (process(ii)Equivalent to :) Roll thickness is reduced by 30% at room temperature.
Molded into a 14mm plate,
Process 6 (Process(iii)1): held at 1200 ° C. for 15 minutes and then water-cooled.
[0068]
  (3) Production method C
  Process 1: Forming round steel with outer diameter of 175mm by hot forging and cutting
Process 2 (Process(i)Equivalent to :) Round steel heated to 1250 ° C,
Process 3: Hot extruded round steel is hot-extruded and formed into a steel pipe having an outer diameter of 64 mm and a wall thickness of 10 mm.
Process 4 (Process(i)): The steel pipe was heated to 1220 ° C. for 10 minutes and then cooled at 1 ° C./second,
Process 5 (process(ii)): Outer diameter after drawing with a cross-section reduction rate of 33% at room temperature
Molded into a steel pipe with a thickness of 50.8 mm and a thickness of 8.5 mm,
Process 6 (Process(iii)): Held at 1210 ° C. for 10 minutes and then water-cooled.
[0069]
[Table 1]

[0070]
About the hot-worked steel plate, cold-rolled steel plate, and cold-worked steel pipe obtained by the said process A, B, or C, the austenite grain size number and the mixing rate were investigated. The austenite grain size number was measured according to the method prescribed in ASTM, and the mixed grain ratio was determined by the method described above. At that time, 20 visual fields were observed in each case.
[0071]
Similarly, from the hot-worked steel sheet, cold-rolled steel sheet and cold-worked steel pipe obtained by the process A, B or C, a creep test piece having an outer diameter of 6 mm and a gauge distance of 30 mm is collected and subjected to a creep test. The creep rupture strength (MPa) at 10,000 ° C. for 10,000 hours and the drawing ratio (interpolated value:%) were examined. The above results are summarized in Table 2.
[0072]
[Table 2]

[0073]
  As can be seen from Table 2, steel (No.2, 4, 6, 7, 9, 11-13, 16 and 18-20), The austenite grain size number and the mixed grain ratio are within the ranges defined by the present invention, regardless of the method of A, B, or C. As a result, the creep rupture strength at 750 ° C. for 10,000 hours is 87 MPa or more, and the drawing ratio is60%It is clear that a heat-resistant pressure-resistant member having high heat-resistance fatigue properties and structure stability can be obtained.
[0074]
No.21 (SUS310) and No.22 (SUS316) have a coarse-grained structure that satisfies the conditions specified in the present invention, but the chemical composition is outside the range specified in the present invention. The strength is remarkably low at 41 MPa and 55 MPa, respectively.
[0075]
For steels whose chemical compositions are outside the range specified in the present invention (Nos. 23 to 29), both the austenite grain size number and the mixed grain ratio are specified in the present invention even when the heat treatment is performed by the production method of the present invention. The coarse grain structure inside is not obtained. As a result, the creep rupture strength is as low as 68 to 78 MPa and the drawing rate is as low as 4 to 23%. No. 25 has an O (oxygen) content that is too high, and No. 26 has an N content that is too high. In No. 29, both the O content and the N content are too high. Since these creep rupture strength and drawing ratio are far below the target values, it is understood that it is important to keep the contents of O and N low. That is, with these comparative steels, a heat and pressure resistant member that exhibits excellent heat fatigue resistance and structural stability in a high temperature range of 700 ° C. or higher cannot be obtained.
[0076]
【The invention's effect】
The austenitic stainless steel of the present invention not only has good high temperature strength and high temperature corrosion resistance, but also has a coarse grain structure with an austenite grain size number and a mixture ratio of 6% or less and 10% or less, respectively, in a high temperature range of 700 ° C or higher. It is suitable as a material for a heat-resistant pressure-resistant member having excellent heat-resistant fatigue properties and structural stability. In addition, the heat and pressure resistant member of the present invention has a high creep rupture strength of 750 ° C. and 10,000 hours of 87 MPa or higher and a draw ratio of 57% or higher, so that the component temperature of a super super critical pressure boiler such as a steam temperature of 700 ° C. or higher Can be used as Furthermore, according to the method of the present invention, the heat and pressure resistant member of the present invention can be produced at low cost.

Claims (8)

In mass%, C: 0.03-0.12%, Si: 0.1-1%, Mn: 0.1-2%, Cr: 20% or more and less than 28%, Ni: more than 35% and 50% Hereinafter, W: 4 to 10%, Ti: 0.01 to 0.3%, Nb: 0.01 to 1%, sol. Al: 0.0005-0.04%, and B: 0.0005-0.01%, the balance is made of Fe and impurities, P as impurities is 0.04% or less, S is 0.010% Hereinafter, Mo has a chemical composition of less than 0.5%, N is less than 0.02%, O (oxygen) is 0.005% or less , an austenite grain size number of 6 or less, and a mixing ratio of 10% or less. An austenitic stainless steel characterized by having a coarse grain structure . A heat and pressure resistant member excellent in heat fatigue resistance and structure stability in a high temperature range, comprising the steel according to claim 1. The heat and pressure resistant member according to claim 2, wherein a creep rupture strength at 750 ° C for 10,000 hours is 80 MPa or more and a drawing ratio is 55% or more. In mass%, C: 0.03-0.12%, Si: 0.1-1%, Mn: 0.1-2%, Cr: 20% or more and less than 28%, Ni: more than 35% and 50% Hereinafter, W: 4 to 10%, Ti: 0.01 to 0.3%, Nb: 0.01 to 1%, sol. Al: 0.0005-0.04%, B: 0.0005-0.01%, and at least one component selected from at least one group from the following first group to the third group, The balance consists of Fe and impurities. P as an impurity is 0.04% or less, S is 0.010% or less, Mo is less than 0.5%, N is less than 0.02%, and O (oxygen) is 0.00. An austenitic stainless steel having a chemical composition of 005% or less, a coarse grain structure having an austenite grain size number of 6 or less and a mixed grain ratio of 10% or less .
First group: 0.0005 to 0.1% Zr in mass%.
Second group: 0.0005 to 0.05% Ca by mass% .
Third group: 0.0005 to 0.2% of rare earth elements, Hf and Pd, respectively, by mass%. A heat and pressure resistant member excellent in heat fatigue resistance and structure stability in a high temperature range, comprising the steel according to claim 4. 6. The heat and pressure resistant member according to claim 5, wherein a creep rupture strength at 750 ° C. for 10,000 hours is 80 MPa or more and a drawing ratio is 55% or more. The heat-resistant fatigue in the high temperature range according to claim 2 or 3, wherein the steel having the chemical composition according to claim 1 is sequentially processed in the following steps (i) , (ii) and (iii): A method for producing a heat and pressure resistant member having excellent characteristics and structural stability.
Step (i) : Heating to 1100 ° C. or more at least once before final processing by hot or cold.
Step (ii) : Plastic working with a cross-section reduction rate of 10% or more is performed.
Step (iii) : Final heat treatment is performed at 1050 ° C. or higher. The heat-resistant fatigue in the high temperature range according to claim 5 or 6, wherein the steel having the chemical composition according to claim 4 is sequentially processed in the following steps (i) , (ii) and (iii): A method for producing a heat and pressure resistant member having excellent characteristics and structural stability.
Step (i) : Heat to 1100 ° C. or more at least once before final processing by hot or cold.
Step (ii) : Plastic working with a cross-section reduction rate of 10% or more is performed.
Step (iii) : Final heat treatment is performed at 1050 ° C. or higher.